In a medium sauce pot, add the washed, peeled, cut potatoes and salt, place over a medium to high heat with lid on. Once they reach a boil point, reduce heat to a low boil. Cook for 15 minutes with lid partially on or until the potatoes are tender when pierced with a fork. Drain and set aside.
While the potatoes are cooking, melt 1 tablespoon of butter in a large skillet. Add the shitake mushrooms and sauté for about 10 minutes, then add the sliced cremini with 1 tablespoon of butter. Continue sautéing for another 15 minutes. At the end of the cooking time, add garlic, salt and pepper, then sauté for 2 more minutes, long enough for the garlic to release it's aroma.
Meanwhile, bring beef broth to a boil in a small sauce pot and mix together starch and a teaspoon or two of water. Then add it to the beef broth, continue boiling until it thickens. Remove from heat.
Add sour cream and cream to potatoes and using a hand beater, whip until smooth and lumps are no longer visible. (I like to warm the cream before adding to the potatoes, microwave for 15 seconds)
Place whipped potatoes on a serving plate, add gravy and mushrooms. Sprinkle a small amount truffle salt over the mushrooms. Garnish with parsley. Enjoy!!